I have the same trouble with my S70. It's just a minor annoyance and all I do is ask the passenger to close the door a little harder, which always works to turn the light off. I could never get it to happen at the dealership when the vehicle was under warranty. Now that it's out of warranty, it's not worth the cost to diagnose and fix, in my opinion.

Just chalk it up as another of the idiosyncracies of the Volvo. A few minor squeaks and rattles have done nothing to dampen my happiness with my S70 purchase, now almost three years old and 73k later.